1. Field of the Invention
The description set forth herein relates generally to a method of producing display capsules. More particularly, the description relates to a method for producing multichromal capsules for use in displays having improved contrast properties and yield.
2. Description of Related Art
Capsules have a variety of uses. Various encapsulation techniques are available to fabricate these capsules. New capsules and encapsulation techniques are desired to expand the applications in which encapsulation technology may be useful. For example, the capsules and encapsulation techniques such as those described in U.S. Pat. No. 5,604,027 to Sheridon, incorporated by reference herein in its entirety, may be used in the manufacture of components for display devices, conventional display devices (some including microcapsules), components for display devices, and the manufacture of such display devices and their components.
Particularly, it is multichromal capsules, such as bichromal beads or twisting particles, in shapes such as balls and cylinders, that are used to make multichromal displays. As used herein the word “bichromal” refers to an item that exhibits at least two colors. Thus, in certain embodiments, the word “bichromal” may include multichromal. Similarly, the word “multichromal” includes bichromal. A bichromal display may be known as electric paper, as described in, for example, U.S. Pat. Nos. 6,703,074; 5,389,945; 4,438,160; 4,261,653; 4,143,103; and 4,176,854, each of which is incorporated by reference herein in its entirety. A bichromal display includes twisting particles that individually rotate to display a desired surface or a desired aspect of transparency of the particles to an observer.
Twisting particles can be, for example, a ball or bead having two distinct hemispheres with both an optical anisotropy and an electrical anisotropy due to each hemisphere surface having a different color and electrical charge. When used in a display, the twisting particles are generally embedded in a fluid-filled cavity of a substrate. The substrate is then generally bonded between glass or plastic sheets to create a multichromal display. When an electric field is presented over the display, the twisting particles within the substrate rotate in the fluid in reaction to the electric field.
In particular, the prior methods of encapsulating multichromal beads require numerous steps in making a display. A multichromal capsule includes a single multichromal bead or ball centered in an oil layer within a capsule that rotates under electrical stimulus. However, under current manufacturing processes some capsules may contain more than one bead (“multiples”), some capsules may have no beads, and/or the beads may be stuck to the capsule wall instead of centered in the oil layer. Capsules that contain two or more balls do not exhibit good ball rotation and impede free bead rotation in response to the switching electric field because of interferences between the balls, causing a loss in contrast of a multichromal display image. Removal of these defective capsules is difficult or impossible, and further results in poor product yields. Thus, insufficient contrast ratio and low yield are two significant problems with encapsulation processes for producing display capsules.
Additionally, other processes for fabricating multichromal display devices include the so called “swollen-sheet” method, in which bare multichromal beads, randomly mixed and dispersed in a silicone elastomeric sheet, are rendered rotatable by swelling the elastomer in silicone oil. Pockets of oil form around each bead, and the beads detach from the elastomer-bead interface. However, one problem of a display device made by this method is that it may exhibit limited environmental stability. For example, the most effective encapsulation process for bichromal beads involves a reaction performed at about 62° C. Charge control additives are incorporated into the beads during manufacturing to enhance the bead's electrical charge and thereby allow for good bead rotation. However, at temperatures above 40° C., the additive may diffuse out of the beads and prevent encapsulation. Thus, the contrast ratio and stability of the capsule commonly degrade at temperatures greater than 40° C.
Accordingly, a need exists for a method for producing display capsules having single multichromal beads encapsulated to provide enhanced contrast properties and a method for producing display capsules that prevents the leaching of additives in single multichromal beads during the encapsulation process.